In the manufacture of foundry molds and cores from sand which is bonded with a curable binder, reclaiming the sand for reuse is an important economic consideration. Foundry sand can normally be used, then reused, repeatedly, when most resin binders are used. Normally there is a small loss of sand on reuse, and this loss is made up by adding pristine sand to the used reclaimed sand. These conditions are experienced with many common resin binders.
In recent years, alkaline phenolic resins have gone into widespread use. These resins can be ester-curable at ambient temperature. Such resin binder systems are disclosed, for example, in U.S. Pat. Nos.: 4,426,467, in which lactones are used as the curing agents; 4,474,904, in which carboxylic acid esters are so used; and 4,468,359, in which the esters are in the gaseous or vapor phase. These patents are expressly incorporated herein by reference.
While these binding systems offer many advantages, there are offsetting disadvantages that are sometimes observed. For example, when reclaimed sand is reused, the tensile strength of the molds or cores drops off. This may have a serious effect on the economics of the foundry.
The extent to which previously used sand is able to be reused is often determined by the tensile properties that can be achieved. One factor which determines the tensile strength is the ability of the binder to bond to the surfaces of the reclaimed sand. Higher rebinding strengths allow higher usage levels of reclaimed sand.
To reclaim sand from a foundry mold or core, the casting is first removed and the used mold or core shaken, vibrated or dismantled mechanically, to loosen the sand and break up any lumps or agglomerates. This sand is then further processed by one of three generally recognized treatment methods for processing reclaimed sand: mechanical; wet; and thermal.
Mechanical treatment processes typically involve subjecting the used sand to grinding, scrubbing or other mechanical attrition to free up the individual grains of sand, removing binder residues, providing clean sand surfaces and removing fines. Wet treatment processes involve washing the sand with water; draining; and drying the washed sand to the moisture levels necessary for subsequent use. Thermal treatment processes involve heating the sand to a temperature of about 1200.degree. F. or above, so that the binder residue is decomposed or burned.
One common objective of these treatment processes is to remove binder residues. The binder residue level may be determined by a loss-on-ignition (L.O.I.) test. Loss-on-ignition is typically determined by heating 10.+-. 1 grams of a sample for about two hours at 1700.degree. F. and measuring residue level. New sand has an L.O.I. of less than about 0.1% to 0.3%, while untreated used sand has an L.O.I. of about 0.5% to 3%.
Where the reclaimed sand is recovered from foundry molds or cores in which the binder was an ester-cured alkaline phenolic resin, wet treatment processes have been found to be very effective. Wet treatment processes can produce sand which exhibits high bonding strength substantially equivalent to that of pristine sand. In contrast, the thermal and mechanical treatment processes do not produce reclaimed sand with such high bonding strength. It is believed that wet treatment processes "cleanse" the surfaces of the reclaimed sand of any harmful residues. While it is not well understood why ester curable alkaline phenolics do not bond well to the surfaces of further mechanically or thermally reclaimed sand previously bonded with ester cured alkaline phenolic binder, some theories have evolved based on apparent facts.
One such fact is that ester curable alkaline phenolic resins do not bond very well to a film, or a residue on a surface, of ester cured alkaline phenolic resin.
Mechanical and thermal treatment processes apparently are not as effective in removing harmful residues as wet treatment processes. Nonetheless, mechanical treatment processes are the most commonly employed by the foundry industry because they are the most economical. Thermal treatment processes are relatively undesirable because of their high energy costs, and wet treatment processes because of disposal problems associated with the wash water and energy costs for drying the sand.
Because of limitations in the mechanical and thermal treatment processes, reclaimed sands so treated, particularly those recovered from foundry molds or cores wherein the cured binder was an ester-cured alkaline phenolic resin, have bonding properties inferior to those of pristine sand. These inferior bonding properties limit the usage level of reclaimed sand in many cases to about 50 weight percent of the total sand when forming foundry cores and molds. Reclaimed sand levels of up to 90 weight percent are needed to achieve the desired economics and minimize disposal costs.
In comparison, the used, reclaimed sand obtained from foundry molds and cores which employ an acid-cured binder, such as acid-cured phenolic resin or furan resins, do not suffer significant losses in bonding strength after thermal or mechanical treatment. However, wet treatment processes are generally not effective in reclaiming such used sand. It is, of course, economically optimal for foundries to be able to reuse as much sand as possible, only buying new sand to make up for relatively minor handling losses. When this is done, foundries can often use as much as 80% to 90% reclaimed sand mixed with 10% to 20% new or pristine sand. However, if the bonding strength of a reclaimed sand is compromised, more than 10% to 20% new sand may have to be mixed with the reclaimed sand in order to increase bonding strength to an acceptable level.
It is desirable to enhance the bonding ability of used sand reclaimed from foundry molds and cores made with ester-cured alkaline phenolic resins, to the extent that usage levels of such used reclaimed sand as high as 80% to 90% by weight can be achieved.
It is known in the art to improve adhesion between polymeric binders and fillers such as sand by using a silane. In U.S. Pat. No. 3,487,043, a composition comprising a furan resin, an acid catalyst, a silane and an insoluble filler material may be polymerized in situ to provide a reinforced resin. The filler may be, for example, fiberglass, titania, ceramic fibers or powders, carbon black, silica, alumina, silica flour, asbestos, clays, or the like. A silane may be applied directly to the filler-reinforcing material, col. 3, lines 55-58. The reinforced or filled resin is used to make pipes, tanks, and other such objects. There is no suggestion of usefulness in the foundry industry.
The treatment of polymer-reinforcing inorganic fillers (glass mats) with a silane solution has been disclosed in U.S. Pat. No. 4,118,540 and UK Patent Specification 882058. In such treatment processes, the filler is immersed in a silane solution, then dried. The silane is believed to improve the affinity or receptivity of the glass mat for the polymer. There is no suggestion of using such a treatment in a foundry application, and the drying step consumes energy.
A number of binder systems are known for use in the foundry industry. J. Robins and others have received several patents relating to the use of isocyanate binder systems in the foundry industry. In one such patent, U.S. Pat. No. 3,403,721, there is a disclosure of precoating the sand with a silane, col. 4, lines 32-39, but, as that patent points out, precoating in this system offers no advantage over adding the silane at some other point.
The addition of silane to resin binder solutions to increase bonding strength is also known. Methods and compositions wherein a silane is added to the resin solution have been disclosed in the two patents just discussed and also in U.S. Pat. Nos. 4,256,623, 4,111,253 and 3,234,259, and UK Patent Specification 876,033.
The addition of a silane solution so as to improve the tensile strength of articles produced from reclaimed sand and an ester-curable alkaline phenolic resin binder is the subject of parent U.S. Pat. application Ser. No. 179,392, filed Apr. 8, 1988, now abandoned, of S.R. Iyer, of which the present application is a continuation-in-part. Iyer observed the decrease in tensile strength that occurred when an ester curable alkali phenolic resin was used as the binder for reclaimed sand that had been mechanically recovered from a shape that had been formed from sand bonded by an ester cured alkali phenolic resin binder, as compared to the tensile strength observed when 100% pristine sand was used.
Following this observation, and realizing its potential effect on the foundry industry, Iyer discovered the ability of mechanically reclaimed sand to form bonded articles of satisfactory tensile strength after treatment of the reclaimed sand with a silane solution. This treatment is generally useful for used sand that has been recovered and subjected to a mechanical or thermal reclamation treatment. It is also useful, but generally is not needed, for a used sand that has been subjected to a wet reclamation treatment.
The Iyer discovery provides a method for treating mechanically and/or thermally reclaimed foundry sand, that had been recovered from used molds or cores formed of sand bonded by ester cured alkaline phenolic resin, for effective reuse with a fresh ester-cured alkaline phenolic resin binder. In this method, the reclaimed sand is mixed with a silane solution in a quantity sufficient to provide an increase in tensile strength in a resin-bonded shape, such as a foundry mold or core produced from the reclaimed, silane-treated sand, using an ester-curable, alkaline phenolic resin.
This discovery provides methods for producing resin-bonded sand shapes, such as foundry molds or cores, wherein at least 20% by weight of the sand used is reclaimed sand that has been treated in accordance with the Iyer invention. In fact, the amount of reclaimed sand preferably is about 50% to 90%, or even 100%, of the sand used. For many foundries, economics dictate that a reclaimed sand level of 60% to 80%, at least, be used.